1. Technical Field of the Invention
The present invention relates to bipolar transistors and, more particularly, to controlling the operation of those transistors in class A mode, according to either a common emitter or common base arrangement. The invention applies advantageously, without limitation, to terminals of wireless communication systems, for example, to cellular mobile telephones.
2. Description of Related Art
A bipolar transistor operating in class A mode according to a common emitter arrangement is customarily used as a transconductor stage, that is to say a voltage/current converter. Such a transconductor is characterized in particular by its linearity and its gain. The linearity can be conveyed by the value, in dBm, of an order 3 interception point (IIP3) for a dual-tone intermodulation test, according to terminology well known to the person skilled in the art. The higher the value of this point, the lower the intermodulation (which is conveyed by the appearance of intermodulation lines) and consequently the better the linearity of the transconductor stage.
During the design of such transconductor stages, the gain and the intermodulation form part of the specified parameters.
In order to improve the linearity of a transconductor stage, it is customary to connect a so-called “degeneracy” resistor between the emitter of the bipolar transistor and earth/ground. The gain gm of the transconductor stage is equal to the gain gm0 of the bipolar transistor proper, divided by (1+gm0RE), where RE denotes the emitter resistance, that is to say the degeneracy resistance. The linearity of the transconductor will improve by increasing this resistance. However, to increase the linearity at constant gm, it is additionally necessary to increase gm0. This value gm0 is thus proportional to the static collector current flowing through the bipolar transistor in the absence of any input signal.
Consequently, the increase in the emitter resistance RE will ultimately be conveyed through an increase in the value of this static collector current, that is to say through an increase in the consumption of current.
Hence, a compromise between linearity and current consumption is envisaged, at present, during the design of such transconductor stages. Also, all the transconductor stages generally exhibit an IcRE product equal to around 100 mV, Ic denoting the static collector current.
The invention aims to afford a solution to this problem.